Star Wars has finally explained why Jedi begin their training learning to block blaster bolts. The Force was introduced in the first Star Wars film, and it wasn't initially developed in any great detail. According to Obi-Wan Kenobi, the Force was the secret to a Jedi's power. "It's an energy field created by all living things," Kenobi explained to the young Luke Skywalker. "It surrounds us and penetrates us. It binds the galaxy together."
Of course, since then the concept of the Force has been expanded upon in countless films, animated spin-offs, and tie-in books and comics. But everything that's since been revealed about the Force has been built on Obi-Wan's words, which introduced viewers to the Force as well as Luke himself. Meanwhile, the prequel trilogy suggested Obi-Wan's lessons with Luke were perfectly in accordance with those of Master Yoda. Obi-Wan began by testing Luke's Force sensitivity against a blaster-firing orb, training him to block its bolts. In Star Wars: Episode II - Attack of the Clones, Yoda was seen using the exact same approach with Jedi Younglings. That's pretty remarkable, given at the time the galaxy was in peace, and even the venerable Jedi Grand Master had no idea the Clone Wars were about to begin.
The recently-published book Secrets of the Jedi has finally explained why Jedi begin their training in this way. This is an in-universe fact-book, supposedly compiled from the notes of Luke Skywalker shortly before his death in Star Wars: The Last Jedi. By this time Luke too has been a teacher of Jedi, and in one section he reflects on the importance of lightsaber training.
"It's said that wielding a lightsaber is less like swinging a sword and more like directing a current of power. When you're attuned to the Force, your thoughts and actions all become part of the same flow of energy, which is then directed through the kyber crystal and into your lightsaber's blade. But that kind of focus takes practice.
In the days of the Jedi Order, younglings would study lightsaber combat from an early age at the Jedi Temple on Coruscant, using low-powered lightsabers designed specifically for training. At first, their primary focus was defense, learning to block laser blasts while wearing special helmets that obscured their sight. By doing so, they could learn to let the Force be their guide. Only after mastering the defensive arts could they begin to study offensive techniques under the tutelage of some of the finest duelists the Jedi Order had to offer."
Luke's explanation makes sense. According to Timothy Zahn's novel Thrawn: Alliances, Jedi are able to swat blaster bolts aside because they have learned to trust in the Force. They experience a moment of so-called "double vision" in which they see both present reality and future threat. This glimpse of the future allows a Jedi to respond, but they have to do so at once. Thus this ability is only possible to someone who has developed a degree of Force sensitivity and who has learned to instinctively trust in the Force. Thus the disciplines involved in this establish the foundation for a Jedi's entire relationship with the Force.
This approach also corresponds with Jedi philosophy as established in Star Wars canon. The first thing a Jedi learns to do with the Force is to use it for defense, not for offense. Jedi are not ready to learn how to use the Force as a weapon until they are comfortable with first considering it a shield - both for themselves and for others.